1. Field of the Invention
The present invention relates to a zoom lens of the rear focus type and an image pickup apparatus using the zoom lens, and more particularly, to a zoom lens of the rear focus type having a back focal distance so long that a color separation prism can occupy the image side space to be used in an image pickup apparatus, such as a video camera and a broadcasting camera, while still maintaining a great increase of the magnification varying range to be achieved at a large relative aperture with the physical length shortened to a compact form, and an image pickup apparatus using such a zoom lens.
2. Description of Related Art
Recently, as home video cameras of improved compact form and reduced weight are being developed, there is seen a remarkable advance in the minimization of the bulk and size of the zoom lens for taking images, too. In particular, great efforts have been devoted to shortening of the physical length, reduction of the diameter of the front lens members and simplification of the form and the construction and arrangement of the constituent parts of the zoom lens.
To attain these ends, one means is to make the focusing provision in a lens unit other than the front or first lens unit. Such a zoom lens is known as the so-called "rear focus type".
The zoom lens of the rear focus type has generally superpassing advantages over the conventional type zoom lens which uses the front lens unit for focusing. For example, the diameter of the front lens members becomes smaller, the compact form of the entire lens system becomes easier to improve, and the minimum object distance can be more easily shortened for close-up photography, particularly supershort focusing. Since the focusing lens unit is smaller in size and lighter in weight, the required driving torque for moving the focusing is reduced, so that rapid focus adjustment can be carried out.
Such a rear focus type of zoom lens is disclosed in, for example, Japanese Laid-Open Patent Applications No. Sho 62-215225, No. Sho 62-206516, No. Sho 62-24213, No. Sho 62-247316 and No. Hei 4-43311. The zoom lens comprises, in order from an object side, a first lens unit of positive refractive power, a second lens unit of negative refractive power, a third lens unit of positive refractive power and a fourth lens unit of positive refractive power, totaling four lens units. The second lens unit axially moves to vary the focal length, while simultaneously moving the fourth lens unit to compensate for the image shift. The fourth lens unit is made also movable for focusing purposes.
Further, Japanese Laid-Open Patent Applications No. Hei 4-43311, No. Hei 4-153615, No. Hei 5-19165, No. Hei 5-27167 and No. Hei 5-60973 have proposed zoom lenses of shortened total length by constructing the fourth lens unit in the form of one or two positive lenses. Another Japanese Laid-Open Patent Application No. Hei 5-60974 has proposed a zoom lens in which the fourth lens unit is composed of positive and negative lenses, totaling two lenses.
In Japanese Laid-Open Patent Applications No. Sho 55-62419, No. Sho 62-24213, No. Sho 62-215225, No. Sho 56-114920, No. Hei 3-200113, No. Hei 4-242707, No. Hei 4-343313 and No. Hei 5-297275, some of their embodiments reveal that each of the third and fourth lens units is composed of a positive lens and a negative lens, totaling two lenses.
In the meantime, video decks have recently enhanced (digitalized) the performance. To meet this trend, the video cameras partake in improving the image quality in various ways. Of these, there is a one which separates the image to original colors by using a color separation optical system. Zoom lenses adapted for such video cameras have been proposed in Japanese Laid-Open Patent Applications No. Hei 5-72474, No. Hei 6-51199, No. Hei 7-199069, No. Hei 7-270684, etc.
In general, the use of the rear focus type in the zoom lens leads to provide possibilities of improving the compact form of the entire system, speeding up the focusing, and assuring reduction of the minimum object distance.
On the other hand, however, with the back focal distance secured to be so long that the color separation prism can be disposed, if it is intended to maintain good stability of aberration correction throughout the entire focusing range for a high optical performance, the lens design becomes very difficult.
Particularly, for a zoom lens that has secured the large relative aperture and the high zoom ratio, it becomes very difficult to simultaneously fulfill the requirements of obtaining the long back focal distance and of obtaining the high optical performance throughout the entire zooming range and throughout the entire focusing range.
Zoom lenses disclosed in Japanese Laid-Open Patent Applications No. Hei 4-26811 and No. Hei 4-88309 have a short back focal distance so that it is difficult to dispose the color separation prism therein.
Zoom lenses disclosed in Japanese Laid-Open Patent Applications No. Hei 4-43311, No. Hei 4-153615, No. Hei 5-19165, No. Hei 5-27167 and No. Hei 5-60973 have a zoom ratio of 6 to 8. If the magnification varying range increases from such a zoom ratio, variation with zooming of chromatic aberrations increases too much greatly. So, it becomes difficult to correct the variation of chromatic aberrations well.
Zoom lenses disclosed in Japanese Laid-Open Patent Applications No. Sho 55-62419, No. Sho 56-114920 and No. Hei 3-200113 have the first or third lens unit made additionally movable for zooming. Therefore, the operating mechanism of the zoom lens becomes so much complicated, causing a problem of increasing the difficulty of improving the compact form.
Zoom lenses disclosed in Japanese Laid-Open Patent Applications No. Hei 4-242707, No. Hei 4-343313 and No. Hei 5-297275 provide for the third lens unit with a large air space. Further, a negative lens in the third lens unit is weak in refractive power. In application to high range zoom lenses, therefore, a problem arises in that the third lens unit produces large chromatic aberrations, which are difficult to correct sufficiently well.
In a zoom lens disclosed in Japanese Laid-Open Patent Application No. Hei 5-297275, a negative lens of meniscus form in the third lens unit has a strong concave surface facing the image side. This structure favors the telephoto form, but the flare component of higher orders a positive lens produces is difficult to correct by the negative lens. Therefore, a problem arises in that the relative aperture and the magnification varying range are hardly taken at the large and high values.
Zoom lenses disclosed in Japanese Laid-Open Patent Applications No. Hei 5-72474, No. Hei 6-51199, No. Hei 7-199069 and No. Hei 7-270684 have a zoom ratio of 10 to 12. In application to the video cameras, this value is not always sufficient.